As is well-known, the catalytic conversion of virgin gas oils containing aromatic, naphthenic and paraffinic molecules results in the formation of a variety of distillates that have ever-increasing utility and importance in the petrochemical industry. The economic and utilitarian value, however, of the residual fractions of the cat cracking processes (also known as cat cracker bottoms) has not increased to the same extent as have the light overhead fractions. One potential use for such cat cracker bottoms is in the manufacture of carbon artifacts. As is well-known, carbon artifacts have been made by pyrolyzing a wide variety of organic materials. Indeed, one carbon artifact of particularly important commercial interest is carbon fiber. Hence, particular reference is made herein to carbon fiber technology. Nevertheless, it should be appreciated that this invention has applicability to carbon artifacts in a general sense, with emphasis upon the production on shaped carbon articles in the form of filaments, yarns, films, ribbons, sheets, etc.
The use of carbon fibers for reinforcing plastic and metal matrices has gained considerable commercial acceptance. The exceptional properties of these reinforcing composite materials, such as their high strength to weight ratio, clearly offset their high preparation costs. It is generally accepted that large scale use of carbon fibers as reinforcing material would gain even greater acceptance in the marketplace, if the costs of the fibers could be substantially reduced. Thus, the formation of carbon fibers from relatively inexpensive carbonaceous pitches has received considerable attention in recent years.
Many materials containing polycondensed aromatics can be converted at early stages of carbonization to a structurally ordered optically anisotropic spherical liquid crystal called mesophase. The presence of this ordered structure prior to carbonization is considered to be fundamental in obtaining a high quality carbon fiber. Thus, one of the first requirements of a feedstock material suitable for carbon fiber production, is its ability to be converted to a highly optically anisotropic material.
In addition, suitable feedstocks for carbon artifact manufacture, and in particular carbon fiber manufacture, should have relatively low softening points and sufficient viscosity suitable for shaping and spinning into desirable articles and fibers.
Unfortunately, many carbonaceous pitches have relatively high softening points. Indeed, incipient coking frequently occurs in such materials at temperatures where they have sufficient viscosity for spinning. The presence of coke, infusible materials, and/or high softening point components, are detrimental to the fiber-making process. Thus, for example, U.S. Pat. No. 3,919,376 discloses the difficulty in deforming pitches which undergo coking and/or polymerization at the softening temperature of the pitch.
Another important characteristic of the feedstock for carbon artifact manufacture is its rate of conversion to a suitable optically anisotropic material. For example, in the above-mentioned U.S. Patent, it is disclosed that 350.degree. C. is the minimum temperature generally required to produce mesophase from a carbonaceous pitch. More importantly, however, is the fact that at least one week of heating is necessary to produce a mesophase content of about 40%, at that minimum temperature. Mesophase, of course, can be generated in shorter times by heating at higher temperatures. However, as indicated above, incipient coking and other undesirable side reactions take place at temperatures in excess of about 425.degree. C.
In U.S. Pat. No. 4,208,267, it has been disclosed that typical graphitized carbonaceous pitches contain a separable fraction which has important physical and chemical properties. Indeed, this separable fraction exhibits a softening range and viscosity suitable for spinning. It also has the ability to be converted rapidly (at temperatures in the range generally of about 230.degree. C. to about 400.degree. C.) to an optically anisotropic, deformable, liquid crystalline material structure. Unfortunately, the amount of separable fraction present in well-known commercially available petroleum pitches, such as Ashland 240 and Ashland 260, to mention a few, is exceedingly low. For example, with Ashland 240, no more than about 10% of the pitch constitutes a separable fraction capable of being thermally converted to a deformable anisotropic phase.
In U.S. Pat. No. 4,184,942, it has been disclosed that the amount of the aforementioned fraction yielding an optical anisotropic pitch can be increased by heat soaking the feedstock at temperatures in the range of 350.degree. C. to 450.degree. C., until spherules visible under polarized light begin to appear.
In U.S. Pat. No. 4,219,404, it has been disclosed that the polycondensed aromatic oils present in isotropic graphitizable pitches are generally detrimental to the rate of formation of highly anisotropic material in such feedstocks when they are heated at elevated temperatures and that, in preparing a feedstock for carbon artifact manufacture, it is particularly advantageous to remove at least a portion of the polycondensed aromatic oils normally present in the pitch simultaneously with, or prior to, heat soaking of the pitch for converting in into a feedstock suitable in carbon artifact manufacture.
More recently, in U.S. Pat. No. 4,271,006 (June 2, 1981), a process has been disclosed for converting cat cracker bottoms to a feedstock suitable in carbon artifact manufacture. Basically, the process requires stripping cat cracker bottoms of fractions boiling below 400.degree. C. and thereafter heat soaking the residue followed by vacuum stripping to provide a carbonaceous pitch.
Cat cracker bottoms like all other heavy aromatic residues obtained from steam cracking, fluid cracking or coal processing are composed of two components: (1) a low molecular weight oil fraction which can be distilled; and (2) an undistillable fraction of high molecular weight. This high molecular weight fraction is insoluble in paraffinic solvents such as n-heptane, iso-octane, pet ether, etc. This fraction is generally called "asphaltene".
It is preferred to use an asphaltene-free feed for the production of pitches. These asphaltenes have a very high molecular weight (up to 10,000), a very high coking characteristic (coking value as high as 67.5 wt% coke yield at 550.degree. C.), and a very high melting point (200.degree.-250.degree. C.).
It is desired to use an asphaltene-free cat cracker bottom. The asphaltene-free cat cracker bottom is free of ash, coke particles and other impurities. The absence of asphaltene, ash, coke particles and other organic and inorganic impurities make the cat cracker bottom distillate an ideal feed for the production of an aromatic pitch with a very high content of liquid crystals. This asphaltene-free cat cracker bottom can be prepared by two methods: (a) by a distillation proces; e.g., vacuum or steam distillation; and (b) by deasphaltenation of the cat cracker bottom. The deasphaltenation can be made readily by solvent extraction with a paraffinic solvent.
In application U.S. Ser. No. 291,990 (filed Aug. 11, 1981) and assigned to a common assignee a process is described for heat soaking a deasphaltenated cat cracker bottom.
In application U.S. Ser. No. 225,060 (filed Jan. 14, 1981) and assigned to a common assignee a process is described for obtaining a feedstock with a low liquid crystal fraction by heat soaking a distillate derived from a cat cracker bottom. The pitch produced in the above applicaton, Ser. No. 225,060 cannot be used directly for carbon fiber production. The liquid crystal fraction has to be extracted from the pitch and used for fiber production.
Whereas, application U.S. Ser. No. 225,060 teaches that all of the cat cracker bottoms can be used to obtain a pitch having low toluene insolubles (Ti), the present invention teaches the opposite, i.e. obtaining a pitch from fractions of the cat cracker bottoms which has a high Ti content (a high content of liquid crystals).
The present invention uses deasphaltenated feedstock fractions to provide a pitch having a high Ti content, and one which does not require Ti solvent extraction prior to spinning into fibers.
The deasphaltenated fractions of a feedstock in accordance with this invention is generally free of ash and impurities, and has the proper rheological properties to allow direct spinning into carbon fibers. The pitch obtained from this fraction produces fibers which have high strength and performance. For example, a deasphaltenated cat cracker bottom fraction obtained in accordance with the present invention, has virtually no coking value at 550.degree. C. compared with a 56% standard coking value for Ashland 240. The deasphaltenated cat cracker bottom fraction is composed of 4, 5, and 6 polycondensed aromatic rings. This provides a uniform feed material which can be carefully controlled to produce a uniform product with a narrow molecular weight distribution.